The present invention relates to a piezoelectric effect device utilizing longitudinal piezoelectric effect, and more specifically relates to a housing structure of the device.
Conventionally, as shown in FIG. 1, such type of the piezoelectric effect device is comprised of a piezoelectric body 300 having a multilayer structure of a sheet or plate made of piezoelectric ceramic material and an internal electrode made of silver-palladium alloy, laminated alternately with one another. A pair of external electrodes are coated to electrically connect to the internal electrodes every one another. A pair of lead wires 600a and 600b are electrically connected to the respective external electrodes. An organic polymer film 13 composed of epoxy resin etc. is overcoated around a peripheral surface of the piezoelectric body 300 except top and bottom faces of the body.
In another prior art as shown in FIG. 2, a piezoelectric body 3 is enclosed by a metal case 9 and a metal stem 7, which are welded together at contact edges 9b to seal the body 3 to thereby improve durability or reliability. The metal stem 7 is provided with a pair of external terminals 8a and 8b supported through hermetic sealers 8c and 8d, and is fixed to one end of the piezoelectric body 3 by means of an adhesive 11. The metal case 9 is also fixed to the other end of the piezoelectric body 3 through another adhesive 11.
With regard to the first-mentioned conventional piezoelectric effect device, an exterior protection is composed of an organic polymer coating which has physical weakness with respect to humidity-resistance due to its permeability and absorptive naturer chemical- or solvent-resistance, and oil-resistance, and which contains contaminant ions, thereby causing spark defect due to reduction of surface resistivity between the internal electrodes, or short defect due to migration of silver component in the internal electrodes.
With regard to the second-mentioned prior art, the metal stem 7 and metal case 9 are generally composed of stainless steel material in view of its good corrosion resistance. However, when curing the adhesive 11 by heating, the piezoelectric body may generate chipping or cracking defect due to thermal stress caused by difference of linear thermal expansion coefficient between the stainless steel material (ten and several ppm/.degree.C.) and the piezoelectric body (about 1 ppm/.degree.C.). Such defect may also occur when driving the device at or below the room temperature.
On the other hand, when using another metal material of a relatively low thermal expansion rate such as Invar alloy (several ppm/.degree.C.) and Super-Invar alloy (about 1 ppm/.degree.C.) instead of stainless steel, there would be caused various drawbacks such as these materials are difficult to process or treat to thereby increase production costs, hermetic seal glass is not formed stably to thereby cause cracking or leaking, and these materials have less corrosion-resistance to thereby degrade reliability of the products.